Flow restrictors in fuel cell flow-field

ABSTRACT

Flow-field for a PEM fuel cell having a plurality of flow-channels including flow-restrictors strategically located throughout to achieve desired pressure differentials between fuel and oxidant supply and exhaust manifolds, and between adjacent flow-channels. A preferred flow-restrictor comprises a constriction in the flow channel that has a cross-sectional area that is less than the cross-sectional area of the flow-channel.

TECHNICAL FIELD

[0001] This invention relates to PEM fuel cells and more particularly tothe reactant flow fields therefor.

BACKGROUND OF THE INVENTION

[0002] Fuel cells have been proposed as a power source for manyapplications. One such fuel cell is the PEM (i.e., Proton ExchangeMembrane) fuel cell. PEM fuel cells are well known in the art andinclude in each cell thereof a so-called “membrane-electrode-assembly”(hereafter MEA) comprising a thin (i.e., ca. 0.0015-0.007 inch),proton-conductive, polymeric, membrane-electrolyte having an anodeelectrode film (i.e., ca. 0.002 inch) formed on one face thereof, and acathode electrode film (i.e., ca. 0.002 inch) formed on the oppositeface thereof. Such membrane-electrolytes are well known in the art andare described in such U.S. Pat. Nos. 5,272,017 and 3,134,697, as well asin the Journal of Power Sources, Volume 29 (1990) pages 367-387, interalia. In general, such membrane-electrolytes are made from ion-exchangeresins, and typically comprise a perfluoronated sulfonic acid polymersuch as NAFION™ available from the E.I. DuPont de Nemours & Co. Theanode and cathode films, on the other hand, typically comprise (1)finely divided carbon particles, very finely divided catalytic particlessupported on the internal and external surfaces of the carbon particles,and proton, conductive material. (e.g., NAFION™) intermingled with thecatalytic and carbon particles, or (2) catalytic particles, sans carbon,dispersed throughout a polytetrafluoroethylene (PTFE) binder. One suchMEA and fuel cell is described in U.S. Pat. No. 5,272,017 issued Dec.21, 1993, and assigned to the assignee of the present invention.

[0003] The MEA is sandwiched between sheets of porous, gas-permeable,conductive material, known as a “diffusion layer”, which press againstthe anode and cathode faces of the MEA and serve as (1) the primarycurrent collectors for the anode and cathode, and (2) mechanical supportfor the MEA. Suitable such primary current collector sheets comprisecarbon or graphite paper or cloth, fine mesh noble metal screen, and thelike, through which the gas can diffuse, or be driven, to contact theMEA underlying the lands, as is well known in the art.

[0004] The thusly formed sandwich is pressed between a pair ofelectrically conductive plates which serve as secondary currentcollectors for collecting the current from the primary currentcollectors, and for conducting current between adjacent cells internallyof the stack (i.e., in the case of bipolar plates), and externally ofthe stack (in the case of monopolar plates at the ends of the stack).The secondary current collecting plates each contain at least one activeregion including a so-called “flow-field” that distributes the fuelcell's gaseous reactants (e.g., H₂ or O₂/air) over the surfaces of theanode and cathode. The flow-field includes a plurality of lands whichengage the primary current collector and define therebetween a pluralityof grooves or flow-channels through which the gaseous reactants flowbetween a supply manifold in a header region of the plate at one end ofthe channel and an exhaust manifold in a header region of the plate atthe other end of the channel.

[0005] The pressure differentials (1) between the supply manifold andthe exhaust manifold, and (2) between adjacent flow channels or segmentsof the same flow channel, are, of considerable importance in designing afuel cell. Serpentine channels have been used to achieve desiredmanifold-to-manifold pressure differentials as well as inter-channelpressure differentials. Serpentine flow-channels have an odd number oflegs extending, in switchback style, between the supply and exhaustmanifolds of the stack. Serpentine flow channels use various widths,depths and lengths to vary the pressure differentials between the supplyand exhaust manifolds, and may be designed to drive some reactant gastrans-land between adjacent channels or between adjacent segments of thesame channel via the current collecting diffusion layer in order toexpose the MEA confronting the land separating the legs to reactant. Forexample, some gas can flow from an upstream leg of a channel (i.e. wherepressure is higher) to a parallel downstream leg of the same channel(i.e. where the pressure is lower) by moving through the diffusion layerengaging the land that separates the upstream leg from the paralleldownstream leg. Non-serpentine flow-channels have been proposed thatextend more or less directly between the supply and exhaust manifolds,i.e. without any hairpin/switchback-type turns therein, and hence inshorter lengths than the serpentine flow-channels. Pressure differentialmanagement is more difficult with non-serpentine flow-channels than withserpentine flow-channels.

[0006] The present invention is directed to a PEM fuel cell flow-fieldthat offers significant design flexibility in achieving desired pressuredifferentials between the supply and exhaust manifolds, and betweenadjacent flow-channels. The invention utilizes flow-restrictionsstrategically located throughout the flow-field to achieve the desiredpressure differentials, and is particularly useful with non-serpentineflow-channels.

SUMMARY OF THE INVENTION

[0007] The present invention relates to a PEM fuel cell of the type thathas (1) a proton exchange membrane having opposing cathode and anodefaces, (2) a gas-permeable, electrically-conductive current collectorengaging at least one of the faces, and, (3) a current-collecting plateengaging the gas-permeable current collector, which current-collectingplate has a gas flow-field thereon that confronts the gas-permeablecurrent collector. The flow-field comprises a plurality of lands thatengage the gas-permeable current collector, and define a plurality ofgas flow-channels through which the gaseous reactants (i.e. H₂ and O₂)flow. The flow-channels each have (a) an inlet end communicating with asupply manifold that supplies a reactant gas to the flow-channels at afirst pressure, and (b) an exit end communicating with an exhaustmanifold that receives the reactant gas from the flow-channels. Inaccordance with the present invention, there is provided: (1) a firstflow-restrictor in a first flow-channel for reducing the first pressureto a second pressure downstream of the first flow-restrictor that isless than the first pressure; and (2) a second flow-restrictor in asecond flow-channel, next adjacent the first flow-channel, formaintaining a third pressure in the second flow-channel upstream of thesecond flow-restrictor sufficiently above the second pressure that itdrives some of the gas from the second flow-channel into the firstflow-channel through the gas-permeable current collector that engagesthe land that separates the two flow-channels. The flow-restrictor willpreferably comprise a constriction in the flow channel that has asmaller cross-sectional area than the flow-channel itself.Alternatively, the flow-restrictor could be a tortuous segment offlow-channel, or ports at the entrance to and exits from theflow-channels that are smaller than the flow-channels themselves. Theflow-restrictors will preferably be located proximate the inlet and exitends of the flow-channels where they can impact the upstream anddownstream pressures over the longest lengths of flow-channel.

[0008] According to a preferred embodiment of the invention, anon-serpentine flow-field has a plurality of flow-channels each of whichhas (a) an inlet leg communicating with the supply manifold, (b) an exitleg communicating with the exhaust manifold, (c) at least one medial legintermediate the inlet and exit legs, (d) a first flow-restrictor in theinlet leg of a first flow-channel for producing a second pressuredownstream of the first flow-restrictor that is less than a firstpressure in the supply manifold, and (e) a second flow-restrictor in theexit leg of a second flow-channel next adjacent the first flow-channelfor maintaining a third pressure in the second flow-channel upstream ofthe second flow-restrictor that is sufficient to drive the gas betweenthe first and second flow-channels through the gas permeable currentcollector that engages the land that separates the two flow-channels.Most preferably, each flow channel has a branched midsection so as toprovide a medial leg that has at least first and second branches, eachof which has a first end communicating with the inlet leg of theflow-channel, and a second end communicating with the exhaust leg of theflow channel. In this context (i.e. a flow-field having branchedmidsection): (i) one embodiment of the invention has theflow-restrictors located only in the inlet and outlet legs of theflow-channels; (ii) another embodiment has the flow-restrictors locatedonly in the branches of the bifurcated midsection; and (iii) in stillanother embodiment, the flow-restrictors are located in both theinlet/outlet legs and in the branches of the furcated midsection.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The invention will be better understood when considered in thelight of the following detailed description of certain specificembodiments thereof which is given hereafter in conjunction with theseveral figures in which:

[0010]FIG. 1 is a schematic, exploded, isometric, illustration of a PEMfuel cell stack (only two cells shown);

[0011]FIG. 2 is an isometric, exploded, view of an MEA and bipolar plateof a PEM fuel cell stack;

[0012]FIG. 3 is an enlargement of a portion of the bipolar plate of FIG.2 where indicated thereon;

[0013]FIG. 4 is a plan view of the bipolar plate of FIG. 2;

[0014]FIG. 5 is an enlarged, isometric view of one embodiment of aflow-restrictor (i.e. a short constriction) in accordance with thepresent invention;

[0015]FIG. 6 is an isometric view of another embodiment of aflow-restrictor (i.e. an elongated constriction) in accordance with thepresent invention;

[0016]FIG. 7 is an enlarged, isometric view of still another embodimentof a flow-restrictor (i.e. tortuous-path) in accordance with the presentinvention;

[0017]FIG. 8 schematically depicts one layout of a flow-field inaccordance with the present invention, but showing only the centerlinesof each of the flow-channels and the locations of the flow-restrictors;

[0018]FIG. 9 schematically depicts another layout of a flow-field inaccordance with the present invention, but showing only the centerlinesof each of the flow-channels and the locations of the flow-restrictors;

[0019]FIG. 10 schematically depicts still, another layout of aflow-field in accordance with the present invention, but showing onlythe centerlines of each of the flow-channels and the locations of theflow-restrictors.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0020] For simplicity, only a two-cell stack (i.e. one bipolar plate) isillustrated and described hereafter, it being understood that a typicalstack will have many more such cells and bipolar plates. FIG. 1 depictsa two-cell, bipolar PEM fuel cell stack having a pair ofmembrane-electrode-assemblies (MEAs) 4 and 6 separated from each otherby an electrically conductive, liquid-cooled, bipolar plate 8. The MEAs4 and 6, and bipolar plate 8, are stacked together between stainlesssteel clamping plates 10 and 12, and monopolar end plates 14 and 16. Theclamping plates 10, 12 are electrically insulated front the end plates14, 16 by a gasket or dielectric coating (not shown). The monopolar endplates 14 and 16, as well as both working faces of the bipolar plate 8,contain a plurality of grooves or channels 18, 20, 22, and 24 defining aso-called “flow field” for distributing fuel and oxidant gases (i.e., H₂& O₂) over the faces of the MEAs 4 and 6. Nonconductive gaskets 26, 28,30, and 32 provide seals and electrical insulation between the severalcomponents of the fuel cell stack. Gas-permeable carbon/graphitediffusion papers 34, 36, 38 and 40 press up against the electrode facesof the MEAs 4 and 6. The end plates 14 and 16 press up against thecarbon/graphite papers 34 and 40 respectively, while the bipolar plate 8presses up against the carbon/graphite paper 36 on the anode face of MEA4, and against carbon/graphite paper 38 on the cathode face of MEA 6.

[0021] The bipolar plates 8 may comprise graphite, graphite-filledpolymer, or metal. Preferably, the bipolar plates will comprise twoseparate metal sheets/panels bonded together so as to provide a coolantflow passage therebetween. Bonding may, for example, be accomplished bybrazing, diffusion bonding, or gluing with a conductive adhesive, as iswell known in the art.

[0022]FIG. 2 is an isometric, exploded view of a bipolar plate 8, firstprimary porous current collector 42, MEA 43 and second primary porouscurrent collector 44 as they are stacked together in a fuel cell. Asecond bipolar plate (not shown) would underlie the second primarycurrent collector 44 to form one complete cell. Similarly, another setof primary current collectors and, MEA (not shown) will overlie theupper sheet 58. The bipolar plate 8 comprises a first exterior metalsheet 58, a second exterior metal sheet 60, and an optional, perforated,interior metal sheet 62 which is brazed interjacent the first metalsheet 58 and the second metal sheet 60. The metal sheets 58, 60 and 62are made as thin as possible (e.g., about 0.002-0.02 inches thick), andmay be formed by stamping, by photo etching (i.e., through aphotolithographic mask) or any other conventional process for shapingsheet metal. The external sheet 58 is formed so as to provide a reactantgas flow field characterized by a plurality of lands 64 which definetherebetween a plurality of non-serpentine gas flow channels 66 throughwhich one of the fuel cell's reactant gases (i.e. H₂) flows from nearone edge 68 of the bipolar plate to near the opposite edge 70 thereof.When the fuel cell is fully assembled, the lands 64 press against theprimary current collectors lying above it (not shown) which, in turn,presses against the MEA with which it is associated (not shown). Inoperation, current flows from the primary current collector through thelands 64 and thence through the stack. The H₂ gas is supplied toflow-channels 66 from a header or supply manifold formed by alignedopenings 72 in the several plates, gaskets, etc., and exits the channels66 via an exhaust manifold formed by aligned openings 74 in the severalplates, gaskets, etc. O₂/air is supplied to the flow-channels on theunderside of plate 60 from a header or supply manifold formed by alignedopenings 76 in the several plates, gaskets, etc., and exhausted throughan exhaust manifold formed by aligned openings 78 in the several plates,gaskets, etc.. Coolant passes between the sheets 58 and 60 from an inletmanifold formed by aligned openings 75 in the several plates, gaskets,etc. to an outlet manifold formed by openings 77 in the several plates,gaskets, etc.. In this regard, the bipolar plate 8 (e.g. see FIG. 2) hasa central active region “A” that engages the primary current collector,and is bordered by inactive header regions “B” and “C”. The activeregion A has a working face having an anode flow field 20 comprising aplurality of flow-channels 66 for distributing hydrogen over the face ofthe MEA 4 that it confronts. A similar working face 22 on the opposite(i.e. cathode) side (not shown) of the bipolar plate 8 serves todistribute air over the face of the MEA 6 that it confronts. The activeregion: A of the bipolar plate 8 is flanked by two inactive headerregions, or border portions, B and C that contain the several openings72, 74, 75, 76, 77 and 78 therethrough. When the plates are stackedtogether, the openings in one bipolar plate are aligned with likeopenings in the other bipolar plates. Other components of the stack suchas gaskets 26, 28, 30 and 32, as well as the membrane of the MEAs 4 and6 and the end plates 14, 16 have corresponding openings (see FIG. 1)that align with the openings 72, 74, 75, 76, 77 and 78 in the bipolarplates in the stack, and together therewith form the aforesaid manifoldsfor supplying and exhausting gaseous reactants and liquid coolantto/from the stack. Referring to FIG. 1, oxygen/air is supplied to theair supply manifold 76 of the stack via appropriate supply plumbing 80,while hydrogen is supplied to the hydrogen supply manifold 72 via supplyplumbing 82. Exhaust plumbing for both the H₂ (84) and O₂/air (86) arealso provided for the H₂ and air exhaust manifolds. Additional plumbing88 and 90 is provided for respectively supplying liquid coolant to, andremoving coolant from, the coolant inlet 75 and outlet 77 manifolds.

[0023] Metal sheet 60 is similar to sheet 58. Like sheet 58, theunderside of the sheet 60 has a working face 22 that engages the firstcurrent collector 42. An optional, perforated, interior, metal sheet 62may be used interjacent the exterior sheets 58 and 60, and, includes aplurality of apertures 92 that cause turbulent flow of the coolant formore effective heat exchange with the exterior sheets 58, and 60respectively. The several sheets 58, 60 and 62 are preferably brazedtogether.

[0024]FIGS. 3 and 4 are, respectively, an enlarged, isometric view ofthe cornerof plate 58 where indicated on FIG. 2, and a plan view ofplate 58 more clearly showing: several flow-restrictors 94 in the inletlegs 96 of the fow-channels 66; the several flow-restrictors 98 in theexit legs 100 of flow-channels 66; and the several flow-restrictors 102in the branches/medial legs 104 and 106 of bifurcated flow-channels 66.In this regard, each flow channel has an inlet leg 96 communicating withthe supply manifold 72, an exit leg 100 communicating with the exhaustmanifold 74; and medial legs/branches 104 and 106, in the midsections ofthe flow-channels, communicating with the inlet and exit legs 96 and 100as more fully described in copending U.S. patent application Ser. No.(Attorney's docket no. GP-303028), that is filed concurrently herewithand is intended to be incorporated herein by reference. The inlet legs96 communicate with the supply manifold 72 via a plurality of openings108 and a slot 110 that communicates with the manifold 72 via apassageway (not shown) that underlies section 112 of the plate 60.Similarly, the exit legs 100 communicate with the exhaust manifold 74via a plurality of openings 114 which in turn communicate with theexhaust manifold 74 via a slot 116 that communicates with the manifold74 via a passageway (not shown) that underlies section 118 of the plate60. The flow-restrictors are strategically positioned/located throughoutthe flow-field, as needed, to achieve desired pressure differentialstherein. Several, but not all, such positionings/locations, arediscussed hereinafter in conjunction with FIGS. 8-10.

[0025] The flow restrictors 94, 98, 102 will preferably compriseconstrictions in the flow channels. In this regard, each flow channel 66has a first cross-sectional area (i.e. transverse the direction of gasflow therein) that predominates throughout most of the length of theflow channel 66, and the constrictions 94, 96, 102 will have a secondcross-sectional area that is less than the first cross-sectional area.Ideally, the several constrictions are sized to result in the same flowrate in all of the medial legs 104, 106 of the flow-channels 66, and thesame flow rate in the inlet 96 and exit 100 legs of the flow-channels66. In some circumstances it may be necessary for one or more of theflow-restrictors to have a different pressure drop than the otherflow-restrictors. Hence one constriction may have a differentcross-sectional area, than the other constrictions. For example,differences between the inlet and outlet flow rates may necessitatemaking the downstream constrictions more severe (i.e. smaller) than theupstream constrictions to achieve the same total pressure drop.

[0026] FIGS. 5-7 depict alternative types of flow-restrictors. FIG. 5depicts a preferred embodiment of a flow-restrictor in accordance withthe present invention, and shows a short constriction 120 in theflow-channel 66. The constriction 122 of FIG. 6 is similar to FIG. 5except that it is elongated to achieve a somewhat greater pressure dropthereacross for the same cross-sectional are as FIG. 5. FIG. 7 depicts aflow restrictor 124 that is a tortuous segment of the flow-channel 66that utilizes extra flow-channel length and multiple hairpin turns 125to provide a desired pressure drop in a short segment of flow-channel66. Another alternative for the inlet 96 and exit 100 legs of theflow-channels 66 is to make the entrance and exit ports 109 and 115 (seeFIG. 4) to/from the flow-channels 66 smaller than the channel itself.

[0027]FIG. 8 is a simplified representation of a flow-field showing only(a) the supply and exhaust manifolds, (b) the centerlines of eachflow-channel, and (c) one embodiment of the placement of flowrestrictors in accordance with the present invention. More specifically,FIG. 8 shows a supply manifold 126, an exhaust manifold 128, and aplurality of flow channels 130 (i.e. only the centerlines thereof shown)extending therebetween. Each flow-channel 130 has an inlet end 132 thatcommunicates with the supply manifold 126, and an exit end 134 thatcommunicates with the exhaust manifold 128. A plurality offlow-restrictors 136, 138 are strategically positioned in theflow-channels 130 to achieve desired pressure differentials throughoutthe flow-field. More specifically yet, a flow restrictor 136 ispositioned near the inlet end 132 of every other flow channel 130 (e.g.the odd numbered flow-channels). Similarly, a flow restrictor 138 isplaced near the exit end 134 of all the other flow-channels 130 (e.g.the even numbered flow-channels). Hence, a first flow-channel 130(a) hasa flow restrictor 136(a) near its inlet end 132, while a next adjacentsecond flow-channel 130(b) has a flow restrictor 138(a) near its exitend 134. A reactant gas is supplied to the flow-channels from the supplymanifold 126 at a first pressure. The flow-restrictor 136 a in the firstflow-channel serves to immediately drop the pressure in the firstflow-channel 130(a) downstream of the flow-restrictor 136 a while thepressure in the second flow-channel 130 b remains essentially the sameas in the supply manifold 126 (i.e. less any losses attributable to thelength of the second flow-channel) which is greater than that in thefirst flow-channel 130 a downstream of flow restrictor 136 a. Propersizing of the flow-restrictors results in: a sufficient pressuredifferential between the first and second flow-channels 130 a, 130 b todrive gas therebetween through the intervening gas-permeable currentcollector; and an equal pressure drop between the inlet 132, and exit134 ends of the first and second flow-channels. The same principlesapply to the remaining sets of adjacent flow-channels of the flow field.

[0028] Like FIG. 8, FIG. 9 is a simplified representation of aflow-field showing only (a) the supply and exhaust manifolds, (b) thecenterlines of each flow-channel, and (c) another embodiment of theplacement of the flow restrictors in accordance with the presentinvention. More specifically, FIG. 9 shows a supply manifold 140, anexhaust manifold 142, and a plurality of flow channels 144 extendingtherebetween. Each flow-channel 144 has: an inlet leg 143 having aninlet end 148 that communicates with the supply manifold 140; an exitleg 150 having an exit end 152 that communicates with the exhaustmanifold 142; and at least one medial leg 146. In the embodiment shown,each flow-channel 144 is bifurcated at its midsection so as to providetwo branches or medial legs 146(a) and 146(b) for each flow channel 144.The medial legs/branches 146(a) and 146(b) each communicate with theinlet and exit legs 143 and 150 for receiving and exhausting a reactantgas from and to the supply 140 and exhaust 142 manifolds, respectively.In this embodiment, flow restrictors 154 are positioned in one of thebranches/medial legs 146(a) near the inlet leg 143 and flow restrictors156 are positioned in another, next adjacent branch 146 b near the exitleg 150. Proper sizing of the flow-restrictors 154, 156 establishes apressure differential between adjacent branches 146 a, 146 b of the samebifurcated flow-channel 144 sufficient to drive reactant gastherebetween through the intervening gas-permeable current collector.The same principles apply to the remaining bifurcated flow-channels ofthe flow field.

[0029]FIG. 10 is a simplified representation of a flow-field showingonly (a) the supply and exhaust manifolds, (b) the centerlines of eachflow-channel, and (c) still another, and preferred, embodiment of theplacement of the flow restrictors in accordance with the presentinvention. More specifically, FIG. 10 depicts a combination of theflow-restrictor placements of the embodiments shown in FIGS. 8 and 9. Inthis regard, flow-restrictors 158 and 160 are positioned in the inletand exit legs 162 and 164, respectively, and flow restrictors 166 and168 are positioned at the beginning of one medial leg 146(a), and at theend of another medial leg 146(b) of the same bifurcated flow-channel144.

[0030] When using flow-restrictors 154, 156 only in the branches 146 a,146 b of the bifurcated midsection (see FIG. 9) of flow-channels 144,none of the inlet 143 and exit 150 legs would have pressuredifferentials. When using flow-restrictors only in the inlet and exitlegs, but not in the branches of a bifurcated flow-channel, half of thebranches would have no pressure differential with their neighbor. Whenusing flow-restrictors 158, 160, 166, 168 in both the inlet/exit legsand in the branches of the bifurcated midsection (see FIG. 10), theinlet 162 and exit 164 legs would have uniform pressure differentials,and half of the bifurcation branches 146 a, 146 b would have morepressure differential than the rest. This is considered to be thepreferred condition since the criteria for pressure differential is thatit should drive enough flow to provide better stack performance thanachievable only by diffusion through the gas-permeable current collectoryet not so much flow that it causes the membranes to dry out.

[0031] Virtually unlimited placement possibilities exist for thelocation of the several flow-restrictors depending on the pressuredifferential profile sought to be achieved by the flow-field designer.Hence, the invention is not limited to the specific embodiments setforth above, but rather only to the extent set forth hereafter in theclaims which follow.

1. A PEM fuel cell comprising (1) a proton exchange membrane havingopposing cathode and anode faces on opposite sides of said membrane, (2)a gas-permeable electrically-conductive current collector engaging atleast one of said faces, and (3) a current-collecting plate engagingsaid gas-permeable current collector and defining a gas flow-fieldconfronting said gas-permeable current collector, said flow-fieldcomprising a plurality of lands engaging said, gas-permeable currentcollector and defining a plurality of gas flow-channels, each of saidflow-channels having (a) an inlet end communicating with a supplymanifold that supplies a reactant gas at a first pressure to all of saidflow-channels, and (b) an exit end communicating with an exhaustmanifold that receives said gas from said flow-channels, a firstflow-restrictor in a first flow-channel to reduce said first pressure toa second pressure downstream of said first flow-restrictor that is lessthan said first pressure, and a second flow-restrictor in a secondflow-channel next adjacent said first flow-channel for maintaining athird pressure in said second flow-channel upstream of said secondflow-restrictor sufficiently above said second pressure to drive saidgas from second flow-channels into said first flow-channel through saidgas-permeable current collector.
 2. A PEM fuel cell according to claim 1wherein said flow-channels each have a first cross-sectional areatransverse the direction of gas flow through said flow-channel, and atleast one of said flow-restrictors comprises a constriction in saidflow-channel having a second cross-sectional area less said than saidfirst cross-sectional area.
 3. A PEM fuel cell according to claim 1wherein at least one of said flow-restrictors comprise a tortuoussegment of said flow-channel.
 4. A PEM fuel cell according to claim 1including a plurality of ports each communicating a said manifold with asaid flow-channel, and at least one of said flow-restrictors is a saidport sized to provide said second and/or said third pressures.
 5. A PEMfuel cell comprising (1) a proton exchange membrane having opposingcathode and anode faces on opposite sides of said membrane, (2) agas-permeable, electrically-conductive current collector engaging atleast one of said faces, (3) a current-collecting plate engaging saidgas-permeable current collector and defining, a gas flow fieldconfronting said gas-permeable current collector, said flow-fieldcomprising a plurality of lands engaging said gas-permeable currentcollector and defining a plurality of gas non-serpentine flow-channels,each of said flow-channels having (a) an inlet leg communicating with asupply manifold that supplies a reactant gas at a first pressure to allsaid flow-channels, (b) an exit leg communicating with an exhaustmanifold that receives said gas from said flow-channels, and (c) atleast one medial leg intermediate said inlet and exit legs, a firstflow-restrictor in the inlet leg of a first of said flow channels forproducing a second pressure downstream of said first flow-restrictorthat is less than said first pressure, and a second flow-restrictor inthe exit leg of a second said flow-channel next adjacent said firstflow-channel for maintaining a third pressure in said secondflow-channel upstream of said second flow-restrictor sufficient to drivesaid gas between said first and second flow-channels through said gaspermeable current collector.
 6. A PEM fuel cell according to claim 5wherein each said flow-channel is branched ed so as to provide a medialleg having at least first and second branches, each having a first endcommunicating with said inlet leg and a second end communicating withsaid exhaust leg.
 7. A PEM fuel cell according to claim 6 wherein saidflow channel is bifurcated and said first branch has a thirdflow-restrictor proximate said first end that reduces the pressure insaid first branch down stream of said third flow-restrictor to a fourthpressure that is below said second pressure, and said second branch hasa fourth flow-restrictor proximate said exit leg for maintaining a fifthpressure in said second branch upstream of said fourth flow-restrictorsufficient to drive said gas between first and second branches throughsaid gas permeable current collector.
 8. A PEM fuel cell comprising (1)a proton exchange membrane having opposing cathode and anode faces onopposite sides of said membrane, (2) a gas-permeableelectrically-conductive current collector engaging at least one of saidfaces, (3) a current-collecting plate engaging said gas-permeablecurrent collector and defining a gas flow-field confronting said gaspermeable current collector, said flow-field comprising a plurality oflands engaging said gas-permeable current collector and defining aplurality of non-serpentine gas flow-channels,, each of saidflow-channels having (a) an inlet leg for receiving gas at a firstpressure from a supply manifold common to all said flow channels, (b) anexit leg for discharging said gas into an exhaust manifold common to allsaid flow-channels, and (c) first and second medial legs intermediatesaid inlet and exit legs, said medial legs each having a first endcommunicating with said inlet leg and a second end communicating withsaid exit leg, said first medial leg having a first flow-restrictorproximate said first inlet leg that reduces the pressure in said firstmedial leg down stream of said first flow-restrictor to a secondpressure that is below said first pressure, and said second medial leghas a second flow-restrictor proximate said exit leg for maintaining athird pressure in said second medial leg upstream of said secondflow-restrictor sufficient to drive said gas between first and secondmedial legs through said gas-permeable current collector.